A mass spectrometer is an apparatus which ionizes sample molecules by electrically charging the sample molecules, separates the generated ions according to their mass-to-charge ratios using an electric field or a magnetic field and measures the amounts thereof as electric current values with a detector. A mass spectrometer is highly sensitive and is superior in quantification performance and identification capability to the conventional analyzers. In recent years, peptide analysis and metabolite analysis, which replace genomic analysis, have received attention in the field of life science, and the effectiveness of amass spectrometer, which is highly sensitive and has excellent identification/quantification capability, has been reassessed.
One of the ionization methods used for amass spectrometer is electrospray ionization method (ESI method) described in PTL 1. In ESI method, a spray unit 101, a counter electrode 102 and a high voltage power supply 103 are used, and a high voltage is applied to a sample solution 104 flowing through the spray unit between the spray unit and the counter electrode to create a Taylor cone 105 and ionize the sample. The ions are detected by a mass analyzer unit provided in the downstream of the counter electrode. ESI method is soft ionization and is thus characterized by being able to ionize samples which easily detach, such as proteins, without destroying their molecular structures. Moreover, since the sample can be introduced continuously using a carrier solvent, ESI method is used for quantitative measurement and structure analysis in the field of biotechnology and the field of drug development by connecting an apparatus for separating components, such as liquid chromatograph, in the upstream of a mass spectrometer.
In general, an actual sample to be measured contains many impurities in addition to the ions to be measured. Thus, the target ions are measured with a mass spectrometer after separating the components using the analysis column of liquid chromatograph. However, when the sample used is a standard substance which does not contain any impurities or when the sample is treated in advance to remove impurities, flow injection analysis method (FIA method) without passing the sample through an analysis column is often employed. In FIA method, a sample is injected into a carrier flow path and introduced to a mass spectrometer by a feeding pump. Advantages of FIA method are that optimization and equilibration of the analysis column are not required because no analysis column is used and that the measurement is thus easy. Also, the solvent used is not restricted by the analysis column, and solvents suitable for ionization, such as methanol, can be used.
An example application of FIA method is the method of PTL 1 and PTL 2. In this method, which involves liquid chromatograph using FIA method, the sample is interposed between bubbles, and thus the loss of the sample during the sample injection can be reduced. Moreover, because the sample diffusion in the flow path can be prevented during the process of sending the solution to the detector such as an ultraviolet spectrophotometer, the S/N ratio is excellent. This method is referred to as sandwich method below.